Whats going on???

I'm 18, and have been in a relationship w/ "K" for over two years. When doing our "thing" we are generally VERY careful, using spermicidal condoms and whatnot. In fact, during the month of april we were actually more careful than usual. However, my period was due the 26th, I usually work like clockwork, and here it is the 28th with no sign of aunt flo! I took a pregnancy test yesterday that came back negative, is that reliable? I have NO symptoms of pregnancy, or even of pms. Besides being a bit bloated and occasionally dizzy, I feel absolutly normal!
Although this pregnancy wasn't planned, it wouldn't be such a bad thing. I would really really like to know, so that I can start taking care of myself and my baby as soon as possible. If I'm not pregnant, which I can't imagine how I could be, I want to know what's going on with my body! ANY advice would be greatly appreciated!